This adds a bit of missed locking around the uploaded info to fix the
concurrent map write.
All the other accesses have locking - this one must have got missed.
This fixes a crash on the google photos backend when an error is
returned from the rest.Call function.
This turned out to be a mis-understanding of the rest docs so
- improved rest.Call docs
- fixed mis-understanding in google photos backend
- fixed similar mis-understading in onedrive backend